The invention relates to an optical path sensor including a filter through which a radiation beam comprising light of at least two wavelengths passes. As the filter is moved perpendicularly to the direction of the optical radiation beam, it causes the attenuations for the beam components of different wavelengths to change in a different ratio.
In an arrangement of this type known from EP-A-93.273 the filter comprises at least two composite filter elements arranged one after the other in the direction of movement, each element only transmitting light of one wavelength. The path dependence is obtained in that, when the filter is moved, different relative portions of the light components of different frequencies reach a detector.
Optical wavelength filters are costly elements. The manufacturing costs of the known elements composed of different filters are considerable.